In general, the wavelength dispersion of an optical fiber is mentioned as a factor to limit an improvement in the bit rate (data transmission rate) of an optical transmission system. The wavelength dispersion is that the propagation speed of an optical signal in an optical fiber differs according to its wavelength, as a result of which there arises a difference in the time at which different wavelength components contained in the optical signal arrive at a receiving end, whereby the waveform of the optical signal being transmitted is caused to distort.
The amount of waveform distortion in this case is proportional to the square of the bit rate, and for example, the amount of waveform distortion at the time of transmitting an optical signal through an optical fiber at a bit rate of 100 Gb/s becomes 100 times with respect to the amount of waveform distortion generated when an optical signal is transmitted through the same optical fiber at a rate of 10 Gb/s. Accordingly, the distance for which an optical signal can be transmitted at a rate of 100 Gb/s by means of an optical fiber of the same optical dispersion property will become 1/100 in the case of an optical signal being transmitted at a rate of 10 Gb/s.
In order to solve the above-mentioned problem resulting from the wavelength dispersion, there has been conventionally used a device to compensate for the dispersion of an optical fiber. In this case, a so-called dispersion compensation fiber having a dispersion property opposite to that of the optical fiber has been widely put in practical use, but the amount of dispersion of the dispersion compensation fiber is fixed, and besides, there arises a new problem that a light amplifier for compensating for optical loss is needed.
Accordingly, as a relatively new conventional method, there has been proposed a dispersion compensation method using digital signal processing (for example, see a first nonpatent document and a second nonpatent document).
A method described in the first nonpatent document is one in which in digital signal processing at a transmitting side, the waveform distortion of a signal is canceled or removed after transmission thereof by calculating a transmitting data series with a transfer function of a property inverse to that of an optical fiber in advance, and it is called a predistortion (or pre-equalization).
In addition, a method described in the second nonpatent document is one in which a transmitting data series is mapped as a symbol of a multiple value by applying orthogonal frequency division multiplexing (OFDM) to optical communications, and is thereafter converted into a plurality of subcarrier modulation signals by carrying out a discrete inverse Fourier transform. In this case, the speed is divided by the number of subcarriers, so a symbol rate is decreased. For example, if the speed is divided by ten subcarriers, the waveform distortion due to dispersion is decreased to 1/100.
Here, reference will be made to the predistortion described in the first nonpatent document, while referring to the attached drawings.
FIG. 5 is a block diagram showing the construction of a conventional digital signal processing optical transmission apparatus using predistortion.
In FIG. 5, the digital signal processing optical transmission apparatus is provided with a digital signal processing (DSP) circuit 2 that calculates predistortion based on an information source 1, D/A converters 3a, 3b that each convert into an analog signal a digital signal which has been calculated in the digital signal processing circuit 2, a laser device (hereinafter referred to as a “laser diode”) 4 that generates laser light, and an optical vector modulator 5 that modulates a real part and an imaginary part independently of each other and outputs an optical transmission signal 6.
Next, reference will be made to the operation of the conventional apparatus as shown in FIG. 5.
The information source 1 is inputted to the digital signal processing circuit 2 while being subjected to parallel expansion, so that it can be easily arithmetically processed into digital signals.
The digital signal processing circuit 2 is composed of a transversal filter which comprises a delay element, a multiplier and an adder, and a look-up table, wherein it performs arithmetic processing with respect to the information source 1, and outputs digital signals.
At this time, in the digital signal processing circuit 2, a convolution arithmetic operation with the inverse function of the dispersion of an optical fiber is carried out for each of a real part and an imaginary part of the information source 1, so that for example, a 6-bit digital signal is calculated, as shown by a plurality of arrows.
The digital signals from the digital signal processing circuit 2 are converted into corresponding analog signals in the D/A converters 3a, 3b, respectively.
Here, the D/A converter 3a converts a digital signal of the real part into a corresponding analog signal, and the D/A converter 3b converts a digital signal of the imaginary part into a corresponding analog signal.
Subsequently, the optical vector modulator 5 modulates the laser light (direct current light) from the laser diode 4 with the individual analog signals from the D/A converters 3a, 3b, and sends out a modulated optical signal to the optical fiber as the optical transmission signal 6.
At this time, the transmission waveform of the optical transmission signal 6 is beforehand made distorted in such a manner that the optical transmission signal 6 can be restored to its original state by the waveform distortion thereof received due to the dispersion of the optical fiber, and hence it is called predistortion.
FIG. 6 is an explanatory view showing the waveform of the optical transmission signal 6 controlled by the conventional digital signal processing optical transmission apparatus (FIG. 5).
FIG. 6(a) is a waveform in cases where the amount of dispersion to be compensated for is large, with peaks of large amplitudes being generated at a certain frequency.
In predistortion, D/A conversion is carried out with a peak of the calculation result being set as an upper limit, so the average value of optical power to be transmitted is caused to change with a ratio of peak power to average power (a peak to average power ratio: PAPR).
In FIG. 6(a), the largest amplitude is assigned to the most significant bit of a digital signal, and an amplitude less than that is assigned to a lower bit thereof, as a result of which average power decreases (the power ratio PAPR is approximately 6).
On the other hand, FIG. 6(b) shows a waveform in cases where the amount of dispersion to be compensated for is zero, and is a waveform itself of an original information source represented by a binary NRZ signal. In this case, average power is a half of peak power.
[First Nonpatent Document]
D. McGhan, et al., “5120 km RZ-DPSK transmission over G652 fiber at 10 Gb/s with no optical dispersion compensation”, OFC/NFOEC2005, PDP27, Anaheim, Calif., March 2005.
[Second Nonpatent Document]
A. J. Lowery, et al., “Performance of optical OFDM in ultra long-haul WDM lightwave systems”, IEEE Journal of Lightwave Technology, vol. 25, no. 1, pp. 131-138, January 2007.